Resistance spot welding is a process used by a number of industries to join together two or more metal workpieces. In particular, spot welding has been used for decades by the automotive, aviation, maritime, railway, and building construction industries, among others, to join together steel workpieces in the manufacture of both load-bearing and non-load-bearing structural assemblies. For example, the automotive industry often uses resistance spot welding to join together pre-fabricated steel workpieces during the manufacture of a vehicle door, hood, trunk lid, or lift gate, as well as during the manufacture of various structural body members included in the vehicle frame. Recently, advances in steel technology have greatly expanded the types and grades of steel that are available to meet any of a wide range of potential end-uses, including those classified (by tensile strength) as high strength steel.
Resistance spot welding, as applicable in the context of steel-to-steel spot welding, relies on the resistance to the flow of an electrical current through overlapping steel workpieces and across their faying interface(s) to generate heat. To carry out such a welding process, a pair of opposed spot welding electrodes are typically pressed under force against facially aligned spots on opposite sides of the workpiece stack-up, which typically includes two or three steel workpieces arranged in lapped configuration, at a predetermined weld site. An electrical current is then passed through the steel workpieces from one electrode to the other. Resistance to the flow of this electrical current generates heat within the steel workpieces and at their faying interface(s). The heat generated at each faying interface initiates a molten steel weld pool that grows and penetrates into each adjacent steel workpiece. The molten steel weld pool eventually solidifies into a weld nugget upon cessation of the electrical current flow. The solidified weld nugget autogenously fuses the workpieces together at the weld site.
The weld schedule that defines the characteristics of the electrical current passed between the welding electrodes has been found to affect the strength—particularly the peel strength—of the final weld nugget. In conventional steel spot welding operations, for instance, the electrical current has typically been passed between the electrodes at a constant current, usually somewhere between 4 kA and 14 kA, for a duration of 150 ms to 1000 ms. A constant-current weld schedule of this kind can in fact initiate and grow a molten steel weld pool at the faying interface(s) of the workpiece stack-up. But as the spot welding process proceeds towards completion and the electrodes further impress into their respective engaged workpiece surfaces, the current density of the flowing electrical current drops as does the power delivered by the electrical current. When this happens, particularly in conjunction with certain grades of high-strength steel that have high carbon contents, the steel alloy weld pool can stop growing and the electrodes, which are typically water-cooled, begin to extract heat from the weld pool faster than the electrical current can generate heat within the weld pool.
The premature drop in current density and power delivery combined with the extraction of heat by the electrodes causes the molten steel weld pool to recede as the outer regions of the weld pool begin to solidify at a relatively slow rate. The ultimately-formed weld nugget thus includes a series of soft, coarse, and alloy deficient shell regions, which are formed during current flow. Typically, these shell regions surround an interior nugget core region formed by way of rapid quenching after the cessation of current flow. The softer shell regions have been found to be more susceptible to crack propagation and tearing, particularly when located near the weld nugget periphery, than the harder interior nugget core region. Certain high-strength steels—such as, for example, steels with a tensile strength of 1000 MPa or greater and in particular those steels having a carbon content of 0.2 wt % or greater—are more likely to contribute to the formation of weld nuggets that include soft, coarse, and alloy deficient shell regions due to their high carbon content and the consequence that such a high carbon content can have on the weldability of steel.